1. Field of the Invention
The present invention is related to optical image rotators which are particularly, but not exclusively, suitable for use with multi-channel optical recording systems.
2. Description of the Related Art
Multi-channel optical tape recording systems utilising polarising reading optics are disclosed in our published European Patent Application No. 0263656 and British Patent Application No. 8720924. In such systems, an optical image rotator is disposed in the path of circularly polarised incident and reflected light beams. A preferred embodiment of rotator disclosed in the aforementioned European Patent Application is a modified form of Vee-block or Abbe rotator having only three metallised reflecting surfaces, and no totally internally reflecting surfaces, in order that the reflections within the rotator have a relatively small polarising effect on light passing through the rotator. While many rotators are optically suitable for use with circularly polarised light, they generally have the disadvantage that they are asymmetric about their axis of rotation, and hence, when rotating at high speed, suffer from dynamic mechanical unbalance. Furthermore, it may be shown that, for good optical performance, the minimum circumscribed diameter of an Abbe or Vee-block rotator about its axis of rotation should be at least double the width of its optical entrance aperture, and such a large circumscribed diameter may be disadvantageous if a compact optical system is required.
In contrast, a Pechan rotator is symmetrical about its axis of rotation and may operate satisfactorily with a considerably lower minimum circumscribed diameter relative to its optical entrance aperture. A Pechan rotator is therefore mechanically preferable to a Vee-block or an Abbe rotator. A conventional Pechan rotator, however has five reflecting surfaces in the optical path, three of which are totally internally reflecting (TIR), and the greater number of reflections would introduce undesirable polarisation effects to an incident circularly polarised light beam, giving rise to an unwanted cyclic amplitude modulation if used in an optical recording system which includes polarising replay optics.
It is an object of the present invention to provide an improved optical image rotator particularly suited for use with polarising optical systems.